Recent clinical observations have suggested that certain antiepileptic drugs may exert significant teratogenic effects in man. We have evaluated the teratogenic potential of several such drugs using the CD-1 mouse as the animal model. The antiepileptic drugs tested fall into three classes, being derivatives of the simple, cyclic imides; hydantoin, succinimide and oxazolidine-2,4-dione. Following intraperitoneal administration on gestational days 8, 9 and 10 or 11, 12 and 13, all the drugs and the three base compounds were embryotoxic and produced a range of congenital malformations. Notable among the abnormalities recorded were a variety of great vessel defects produced by many of the compounds with particularly high incidences being induced by Trimethadione. There was good agreement between the types of malformations produced in mice with those reported following human exposure indicating the CD-1 mouse is a good model to study teratogenic actions of this class of compounds. In addition, the common teratogenic potential and the structural similarities between these chemicals suggest a common receptor or a mutual mechanism of action.